Prior drop-on-demand ink jet print heads typically eject ink drops of a single volume that produce on a print medium dots of ink sized to provide "solid fill" printing at a given resolution, such as 12 dots per millimeter. Single dot size printing is acceptable for most text and graphics printing applications not requiring "photographic" image quality. Photographic image quality normally requires a combination of high dot-resolution and an ability to modulate a reflectance (i.e. gray scale) of dots forming the image.
In single dot size printing, average reflectance of a region of an image is typically modulated by a process referred to as "dithering" in which the perceived intensity of an array of dots is modulated by selectively printing the array at a predetermined dot density. For example, if a 50 percent local average reflectance is desired, half of the dots in the array are printed. A "checker-board" pattern provides the most uniform appearing 50 percent local average reflectance. Multiple dither pattern dot densities are possible to provide a wide range of reflectance levels. For a two-by-two dot array, four reflectance level patterns are possible. An eight-by-eight dot array can produce 256 reflectance levels. An usable gray scale image is achieved by distributing a myriad of appropriately dithered arrays across a print medium in a predetermined arrangement.
However, with dithering, there is a trade-off between the number of possible reflectance levels and the dot array area required to achieve those levels. Eight-by-eight dot array dithering in a printer having 12 dot per millimeter (300 dots per inch) resolution results in an effective gray scale resolution as low as 1.5 dots per millimeter (75 dots per inch). Gray scale images printed with such dither array patterns, however, suffer from image quality.
An alternative to dithering is ink dot size modulation that entails controlling the volume of each drop of ink ejected by the ink jet head. Ink dot size modulation (hereafter referred to as "gray scale printing") maintains full printer resolution by eliminating the need for dithering. Moreover, gray scale printing provides greater effective printing resolution. For example, an image printed with two dot sizes at 12 dots per millimeter (300 dots per inch) resolution may have a better appearance than the same image printed with one dot size at 24 dots per millimeter (600 dots per inch) resolution with a two-dot dither array.
There are previously known apparatus and methods for modulating the volume of ink drops ejected from an ink jet print head. U.S. Pat. No. 3,946,398, issued Mar. 23, 1976 for a METHOD AND APPARATUS FOR RECORDING WITH WRITING FLUIDS AND DROP PROJECTION MEANS THEREFORE describes a variable drop volume drop-on-demand ink jet head that ejects ink drops in response to pressure pulses developed in an ink pressure chamber by a piezoceramic transducer (hereafter referred to as a "PZT"). Drop volume modulation entails varying an amount of electrical waveform energy applied to the PZT for the generation of each pressure pulse. However, it is noted that varying the drop volume also varies the drop ejection velocity which causes in drop landing position errors. Constant drop volume, therefore, is taught as a way of maintaining image quality. Moreover, the drop ejection rate is limited to about 3000 drops per second, a rate that is slow compared to typical printing speed requirements.
U.S. Pat. No. 4,393,384, issued Jul. 12, 1983 for an INK PRINTHEAD DROPLET EJECTING TECHNIQUE describes an improved PZT drive waveform that produces pressure pulses which are timed to interact with an ink meniscus positioned in an ink jet orifice to modulate ink drop volume. The drive waveform is shaped to avoid ink meniscus and print head resonances, and to prevent excessive negative pressure excursions, thereby achieving a higher drop ejection rate, a faster drop ejection velocity, and improved drop landing position accuracy. The technique provides independent control of drop volume and ejection velocity.
However, this droplet ejection technique only provides ink drops having a diameter equal to or larger than the orifice diameter. An orifice diameter ink drop flattens upon impacting a print medium, producing a dot larger than the orifice diameter. Solid fill printing entails ejecting a continuous stream of the largest volume ink drops tangentially spaced apart at the resolution of the printer. Therefore, in a 12 dot per millimeter resolution printer, the largest dots must be about 118 microns in diameter. If gray scale printing is required, smaller dots are required that are limited to a diameter somewhat larger than the orifice diameter. Clearly, an orifice diameter approaching 25 microns is required, but this is a diameter that is impractical to manufacture and which clogs easily.
U.S. Pat. No. 5,124,716, issued Jun. 23, 1992 for a METHOD AND APPARATUS FOR PRINTING WITH INK DROPS OF VARYING SIZES USING A DROP-ON-DEMAND INK JET PRINT HEAD, assigned to the assignee of the present invention, and U.S. Pat. No. 4,639,735, issued Jan. 27, 1987 for APPARATUS FOR DRIVING LIQUID JET HEAD describe circuits and PZT drive waveforms suitable for ejecting ink drops smaller than an ink jet orifice diameter. However, each ink drop has an ejection velocity proportional to its volume which, unfortunately, can cause drop landing position errors.
Ink drop ejection velocity compensation is described in copending U.S. Pat. application Ser. No. 07/892494 of Roy et al., filed Jun. 3, 1992 for METHOD AND APPARATUS FOR PRINTING WITH A DROP-ON-DEMAND INK-JET PRINT HEAD USING AN ELECTRIC FIELD and assigned to the assignee of the present invention. A time invariant electric field accelerates the ink drops in inverse proportion to their volumes, thereby reducing the effect of ejection velocity differences. In another aspect of electric field operation, a PZT is driven with a waveform sufficient to cause an ink meniscus to bulge from the orifice, but insufficient to cause drop ejection. The electric field attracts a fine filament of ink from the bulging meniscus to form an ink drop smaller than the orifice diameter. Unfortunately, the electric field adds complexity, cost, potential danger, dust attraction, and unreliability to a printer.
And yet another approach to modulating drop volume is disclosed in U.S. Pat. No. 4,746,935, issued May 24, 1988 for a MULTITONE INK JET PRINTER AND METHOD OF OPERATION. This describes an ink jet print head having multiple orifice sizes, each optimized to eject a particular drop volume. Of course, such a printhead is significantly more complex than a single orifice size print head having at least two times the number of jets, and still requires a very small orifice to produce the smallest drop volume.
U.S. Pat. No. 4,503,444, issued Mar. 5, 1985 for a METHOD AND APPARATUS FOR GENERATING A GRAY SCALE WITH A HIGH SPEED THERMAL INK JET PRINTER, U.S. Pat. No. 4,513,299, issued Apr. 23, 1985 for SPOT SIZE MODULATION USING MULTIPLE PULSE RESONANCE DROP EJECTION, and "Spot-Size Modulation in Drop-On-Demand Ink-Jet Technology," E. P. Hofer, SID Digest, 1985, pp. 321, 322, each describe using a multi-pulse PZT drive waveform to eject a predetermined number of small ink drops that merge during flight to form a single larger ink drop. This technique has the advantage of constant drop ejection velocity, but inherently forms drops much larger than the ink jet head orifice diameter.
Clearly, the physical laws governing ink jet drop formation and ejection are complexly interactive. Therefore, U.S. Pat. No. 4,730,197, issued Mar. 8, 1988 for an IMPULSE INK JET SYSTEM describes and characterizes numerous interactions among ink jet geometric features, PZT drive waveforms, meniscus resonance, pressure chamber resonance, and ink drop ejection characteristics. In particular, in a multiple-orifice print head, cross-talk among the jets affects ink drop volume uniformity, so "dummy channels" and compliant chamber walls are provided to minimize the effects of cross-talk. Drop ejection rates of 10 kilohertz are achieved with PZT drive waveform compensation techniques that account for print head and fluidic resonances. However, this reference strives to achieve uniform drop volume so that the resulting drop diameter is about the same as the orifice diameter. There is no recognition of ink drop volume modulation in the patent, and the patent is not addressed to gray scale printing.
U.S. Pat. No. 5,170,177, issued Dec. 8, 1992 for a METHOD OF OPERATING AN INK JET TO ACHIEVE HIGH PRINT QUALITY AND HIGH PRINT RATE, assigned to the assignee of the present invention, describes PZT drive waveforms having a spectral energy distribution that is minimized at dominant ink jet head resonant frequencies. A constant ink drop volume and ejection velocity are thereby achieved over a wide range of drop repetition rates. However, similar to the teaching of U.S. Pat. No. 4,730,197, uniform and optimum ink drop volume is sought, and the resulting drop diameter is about the same as the orifice diameter. Again, there is no recognition of ink drop volume modulation nor is attention given to gray scale printing.
What is needed, therefore, is a simple and inexpensive ink jet print head system that provides high-resolution gray scale printing without sacrificing performance. This need is met by the design and method of the present invention.